1. Field of the Invention
This invention relates to a process for producing of laminated sheets which have excellent electrical characteristics such as insulation resistance, dielectric constant or dielectric dissipation factor and heat resistance and which are excellent, in particular, in punchability and more particularly, it relates to a process for the production of laminated sheets comprising a base paper using wood pulp having an .alpha.-cellulose content of 87.0% or more.
2. Description of the Prior Art
Laminated sheets are produced by impregnating a base paper suitable for laminated sheet with a synthetic resin and molding thereof and are widely used for industrial and home electronic equipments. In recent years, printed wiring boards have come to be used under more stringent conditions due to substantial developments in the electronic industry. Hence there has been a strong increase in requests for specific physical properties of laminated sheets. In particular, laminated sheets having good punchability in response to the increased demand for higher density printed wirings these days.
Base papers for laminated sheets which are widely used at present made from wood pulp as a raw material, are obtained from a bleached kraft pulp having an .alpha.-cellulose content of 83 to 86%. Formation of cracks around punched holes or delamination is found in laminated sheets using a base paper as a substrate and hence, laminated sheets requiring punchability cannot be obtained therefrom. When the punching processing is conducted at high temperatures the punching shearing force of the laminated sheet becomes small whereby the punchability can be improved. However, expansion and shrinkage accompanying the heating and cooling steps become high, thereby the resulting laminated sheet is no longer suitable for use in high density printed wiring where dimensional stability is required. Another case is that of cotton linter papers which are base papers for laminated sheets using non-wood pulp. These papers have relatively good punchability but have the added problem of a dimensional stability and are therefore are not suitable for use in laminated sheets for high density printed wiring. Further, the cotton linter papers suffer from yearly price fractuation and lack of market stability in regards to their supply. Thus the appearance of a base paper which is produced from wood pulps of stabilized supply and which can meet the above requirements is desired.
On the other hand, investigations on the punchability have also been made with respect to various plasticizations of synthetic resins. However, in view of problems in the electric characteristics and heat resistance thereof, thoroughly satisfactory results have not yet been obtained.
In order to overcome the above described drawbacks, the present inventors have extensively studied the subject matter and found that, if wood pulps having an .alpha.-cellulose content of 87.0% or more are used for producing base papers, laminated sheets having excellent punchability and excellent electric characteristics and heat resistance can be obtained as described by the present invention.